The present invention generally relates to image transfer technology and, more particularly, to an apparatus and method for charging imaging members of image transfer devices during the printing process, and an image transfer device utilizing the apparatus and method.
As used herein, the term “image transfer device” generally refers to all types of devices used for creating and/or transferring an image in an electrophotographic process, including laser printers, copiers, facsimiles, and the like. As used herein, the term “electrophotographic process” includes both dry and liquid electrophotographic (LEP) processes.
In an electrophotographic image transfer device, the surface of a photoconducting material (i.e., a photoreceptor) is charged to a substantially uniform electrostatic potential so as to sensitize the surface. An electrostatic latent image is created on the surface of the charged photoconducting material by selectively exposing areas of the photoconductor surface to a light image of the original document being reproduced. A difference in electrostatic charge density is created between the areas on the photoconductor surface exposed and unexposed to light. For example, in a liquid electrophotographic process, the photoconductor surface is initially charged to approximately −1000 Volts, with the exposed photoconductor surface discharged to approximately −50 Volts. Alternatively, the photoconductor surface can be initially charged to 1000 Volts, with the exposed surface discharged to approximately 50 Volts.
The electrostatic latent image on the photoconductor surface is developed into a visible image using electrostatic toners or pigments. The toners are selectively attracted to the photoconductor surface either exposed or unexposed to light, depending on the relative electrostatic charges of the photoconductor surface, development electrode, and toner. The photoconductor surface may be either positively or negatively charged, and the toner system similarly may contain negatively or positively charged particles.
A sheet of paper or other medium is passed close to the photoconductor surface, which may be in the form of a rotating drum or belt, transferring the toner from the photoconductor surface onto the paper in the pattern of the image developed on the photoconductor surface, thereby forming a hard image. The transfer of the toner may be an electrostatic transfer, as when the sheet has an electric charge opposite that of the toner, or may be a heat transfer, as when a heated transfer roller is used, or a combination of electrostatic and heat transfer. In some printer embodiments, the toner may first be transferred from the photoconductor surface to an intermediate transfer medium, and then from the intermediate transfer medium to a sheet of paper.
Charging of the photoconductor surface may be accomplished by any of several types of charging devices, such as a corotron (a corona wire having a DC voltage and an electrostatic shield), a dicorotron (a glass covered corona wire with AC voltage, and electrostatic shield with DC voltage, and an insulating housing), a scorotron (a corotron with an added biased conducting grid), a discorotron (a dicorotron with an added biased conducting strip), a pin scorotron (a corona pin array housing a high voltage and a biased conducting grid), or a charge roller. A charge roller has been observed to provide adequate electrical charge with the associated benefits of saving space along a surface of the photoconductor, as well as a reduction in maintenance.
Charge rollers having a variety of designs are known in the art. Some charge rollers are made of a conductive elastomeric material, commonly urethane, molded over a metal core. The charge roller may be lightly pressed against a photoconductor surface to maintain a constant footprint and therefore provide more consistent charging. In some configurations, steady pressure against the photoconductor surface may not be desirable to maintain a constant footprint. For example, some photoconductor configurations have a seam region wherein a photoconductor sheet is inserted into a drum surface and attached. The drum may have a depression in the seam region so that the seam region of the photoconductor does not protrude farther than the radius of the photoconductor and be subject to mechanical damage by contact with other image transfer device components. However, in a liquid electrophotographic implementation, imaging oil may accumulate within the seam region of the photoconductor. Print defects may result in the hard image if the charge roller picks up oil from the seam region and deposits it on other portions of the surface of the photoconductor. In dry electrophotographic processes, potentially damaging electrical arcing may occur if the charge roller enters the seam region of the photoconductor. Accordingly, it is desirable to reduce or avoid entry of the charge roller into the depressed seam region.
However, if the charge roller comes out of contact with the photoconductor and does not enter the seam region, then the photoconductor is not charged in the seam region. Not charging the photoconductor in the seam region is equivalent to exposing the photoconductor with light, so during development of the latent image, toners and pigments are undesirably attracted to the uncharged seam region. Although the toners and pigments are cleaned from the seam region, there is a resultant wasting of materials that increases the cost of printing.